The present invention generally relates to a feeding arrangement for sheets such as copy paper sheets and the like and more particularly, to an automatic sheet feeding apparatus for use, for example, in a copying machine, facsimile equipment, printer, etc.
Conventionally, there has been provided, for example, an automatic copying sheet feeding apparatus for use in an electrophotographic copying machine as shown in FIG. 1. The automatic copying sheet feeding apparatus of FIG. 1 comprises a platform or hopper base 1 for accommodating therein a stack of copying sheets 6, a driven roller 2 arranged to rotate clockwise for feeding a single uppermost sheet with or without a few other sheets thereunder from the hopper base 1, and a sheet separating means M, including an upper roller 3 for forward rotation serving as a feeding roller which is made of normal rubber material and which rotates in the direction of advance of the sheet and rotates to feed the single uppermost sheet towards a pair of conveyance rollers 5 and a lower roller 4 for reverse rotation serving as a separating roller which is made of expanded rubber material and which rotates in the direction opposite to movement of the sheets to prevent other sheets below the single uppermost sheet from moving forward beyond the separating means. The feeding roller 3 has its peripheral surface provided with a material having a coefficient of friction larger than that of the separating roller 4.
However, the automatic sheet feeding apparatus as described above has disadvantages as follows.
Since the rollers 3 and 4 are made of normal rubber material or expanded rubber material, as described above, these rollers 3 and 4 can sometimes repel each other so that the separating roller 4 oscillates or moves upwardly and downwardly with respect to the upper roller 3, thus resulting in variation of contact pressure against the feeding roller 3 and generation of noise, with consequent reduction in the sheet feeding force, and instability in the sheet separating function. Under the above circumstances, there may arise such inconvenience that some sheets fail to reach the conveyance rollers 5 or become jammed around the separating means, and also that two or more sheets may be fed to the conveyer rollers 5 beyond the separating means at the same time. The problem as described above can not be solved merely by strengthening the contact pressure between the rollers 3 and 4.
If the contact pressure between the rollers 3 and 4 is increased the oscillation of the separating roller 4 becomes more violent, while the peripheral surface of the separating roller 4 is abraded excessively. Additionally the sheets, if they are very thin, may become winkled or folded.
Under such a circumstance as described above, if the driven roller 2 is kept rotating after the leading edge of the sheet has reached the rollers 3 and 4, the sheet forms a loop between the driven roller 2 and the separating means or the leading edge of the few other sheets fed with the single uppermost sheet are folded and jammed up in the space between the driven roller 2 and the separating means M. In order to avoid such a problem, it is necessary to provide a mechanism for stopping the rotation of the driven roller 2 after the leading edges of the sheets have reached the rollers 3 and 4, or additionally to incorporate into the driven roller 2 a unidirectional clutch which allows the driven roller 2 to rotate freely in the clockwise direction with respect to its rotating shaft. However, such a mechanism results in the complication of the sheet feeding apparatus.
It is to be noted that, in a specific type of an electrophotographic copying machine which is designed so as to transfer different toner images onto both sides of a sheet, the automatic sheet feeding apparatus therein is arranged so as to feed a sheet which has been transferred with a toner image on its one side for transferring another image onto its other side. Accordingly, some toner of the toner image transferred to the one side of the sheet unavoidably adheres to the peripheral surface of the feeding roller 3, so that the coefficient of friction of the roller is reduced by 20 to 50%. As a result, in such a type of apparatus, the sheet feeding force, as described above, is considerably reduced.
Conventionally, in order to avoid such problems as described above, there have been provided improved apparatuses as follows.
(a) In one of such improved apparatus, the conveyance rollers 5 are located as close to the rollers 3 and 4 as possible in order to receive the sheet from the separating means as soon as possible.
(b) In another improved apparatus, the driven roller 2 is movable away from the sheet at the time when the leading edge of the sheet has reached the feeding roller 3 in order to reduce or eliminate the resistance of the roller 2 against the sheet.
(c) In a further improved apparatus, the feeding roller 3 and the separating roller 4 are respectively divided into two or more parts in the axial direction thereof and arranged in such a manner that each part of the feeding roller 3 extends slightly into the space between the adjacent two parts of the separating roller. This type of apparatus is provided, for example, in Japanese Patent Publication Tokkaisho No. 50-40,603.
(d) In a still further improved apparatus, a minor clearance is formed between the rollers 2 and 3. This type of apparatus is provided, for example, in Japanese Laid Open Utility Model Application Jikkaisho No. 54-51,786.
These improved apparatuses as described above, however, respectively have the following drawbacks.
In the apparatus of the item (a), it is necessary to provide a number of conveyance rollers 5 in a limited space. In the apparatus of the item (b), it is necessary to provide a complicated mechanism for moving the hopper base 1 or the driven roller 2 upwardly and downwardly relatively each other. Such complicated mechanism may cause the deterioration of the reliability of the essential function of the driven roller 2 which is to feed the sheets forward one by one. In the apparatus of the item (c), it is very difficult to adjust the desired distance between the axes of the rollers 3 and 4. In the apparatus of the item (d), it is very difficult to adjust the width of the clearance, in other words, the contact pressure of each roller 3 and 4 with the sheet to be supplied so as to exactly correspond to the thickness of the sheet, especially for a thin and flexible sheet. In the event that the adjustment of the clearance is improper, the roller 3 and 4 will fail to contact the sheet under a suitable pressure. As a result, two or more sheets, may be frequently fed by the feeding roller 3 and a sheet may be jammed up around the rollers 3 and 4 in many cases.